1. Field of the Invention
The present invention relates to an installation for the formation of a laser beam suitable for isotope separation in an atomic vapor (SILVA process).
2. Discussion of the Background
Isotope separation by laser in an atomic vapor is based on the selective photoionization of uranium 235 atoms by resonant absorption. This process requires the production of a laser beam having special properties. It must firstly be polychromatic, i.e. formed from three or four colors, in order to permit the excitation of the various transitions involved in the photoionization process. It must also be wavelength tunable between approximately 550 and 700 nm, to permit the adjustment of the various wavelengths to the transitions involved. It must have a good spectral fineness close to 10.sup.-6. It must supply pulses with a duration of approximately 50 ns with a repetition rate of approximately 10 kHz. Finally, its average power must be a few dozen Watts for laboratory installations and must be able to reach 25 kW for an industrial installation.
In order to achieve such performance characteristics, installations have been constructed having several dye laser chains (i.e. tunable lasers), operating at different wavelengths. Each chain comprises an oscillator stage and an amplifier stage connected in series. Each of the stages of said chains is optically pumped by a chain of lasers arranged in series, each of the pumping lasers operating at a high average power (several hundred Watts). Each pumping chain supplies a bean, whose average power is approximately 1 kW.
The various beans emitted by the different pumping chains are directed by appropriate optical elements (mirrors, lenses, etc.) towards the stages of the dye lasers. The lasers forming the pumping chains are generally copper vapor lasers, which emit two lines respectively at 510 and 578 nm with a repetition rate of approximately 5 kHz.
FIG. 1 shows an installation according to this prior art. The dye laser chain comprises an oscillator stage 10 and amplifier stages 12, 14, 16. There are the sane number of chains of this type as there are wavelengths to be produced (e.g. four).
In the illustrated example, the pumping chain comprises four chains A, B, C and D, each constituted by an oscillator 20 and three amplifiers 22, 24, 26. The four beans 30, 32, 34, 36 supplied are controlled and multiplexed in a system 40 and are then transported by a set of mirrors and lenses and are finally supplied to two bean distribution systems 44 and 46 positioned on either side of the dye laser chain.
It is not appropriate here to describe the wavelength multiplexing means of the beans finally supplied by the various dye laser chains or the separator where the selective photoionization of the uranium 235 finally takes place, because these means are well known and do not form part of the invention. A description is provided in the work entitled "Le Laser-Principes et techniques d'application", coordinator H. MAILLET, chapter 4: "Separation isotopique par laser" by M. CLERC, P. RIGNY and O. DE WITTE (Edition Lavoisier).
Although satisfactory in certain respects, these installations suffer from disadvantages.
Firstly, they have considerable overall dimensions as a result of the use of copper vapor lasers, which are several meters long and have voluminous electrical supply circuits. This is in particular due to the fact that these lasers use a low pressure gas as the active medium. They also require numerous optical elements (mirrors, lenses, etc.) in order to control, multiplex and direct the different light beans emitted by the pumping chains to the dye chains. Therefore, numerous mechanical constraints appear for ensuring the correct focusing of the pumping beans in the dye lasers. These mechanical constraints are made worse by the fact that the two subsystems, namely that of the dye chain (at the top of FIG. 1) and that of the pumping chain (at the bottom of FIG. 1) are in practice very remote from one another and are usually positioned in separate rooms or even buildings, due to the widely differing technologies involved.
Finally, the increase of the average power, which is always desirable when it is wished to achieve industrial dimensions, causes serious problems as a result of the power limiting phenomena occurring in pumping chains, linked with the performance characteristics of the optics (in-line losses and flux resistance threshold) and the saturation of the medium.